Motor drive device including insulation deterioration detection function and insulation resistance detection method of motor

ABSTRACT

A motor drive device of the present invention includes: a rectification circuit configured to rectify an AC voltage; a power source unit configured to smooth a DC voltage by a capacitor; an inverter unit configured to drive a motor by converting a DC voltage into an AC voltage; a current detection unit configured to measure a current flowing through a resistor which is connected to a coil of the motor and the capacitor; a voltage detection unit configured to measure a value of a voltage across the capacitor; a second switch that grounds the capacitor; and an insulation resistance detection unit configured to detect an insulation resistance value of a motor by using two sets of a current value and a voltage value measured in two states where the second switch is turned off and turned on.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a new U.S. patent application that claims benefit ofJP 2014-001812, filed on Jan. 8, 2014, the entire content of JP2014-001812 is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a motor drive device and an insulationresistance detection method of a motor, and in particular, to a motordrive device including an exact motor insulation resistance measurementfunction that has removed the influence of a leakage current flowingthrough a semiconductor switching element of an inverter and aninsulation deterioration detection function, and an insulationresistance detection method of a motor.

BACKGROUND OF THE INVENTION

Conventionally, a motor drive device including a function for detectinginsulation deterioration of a motor winding (coil) by applying a voltagecharged across a smoothing capacitor of a DC link unit between the motorwinding and the ground is known (e.g., Japanese Patent Publication No.JP-B-4554501). In a conventional motor drive device, insulationdeterioration of a motor is detected by measuring a leakage currentflowing between a motor coil and the ground by applying a voltagecharged across a smoothing capacitor of a direct-current power source(DC link unit) connected to an inverter between the motor coil and theground after shutting off an alternating-current power source by aswitch.

Further, a motor drive device including a plurality of inverter unitsfor driving a plurality of motors is known, which calculates aninsulation resistance of each motor by detecting voltages and currentsat a time at the same timing by using one voltage detection unit of acommon converter unit and a plurality of current detection units of eachinverter unit for each motor (e.g., Japanese Patent Publication No.JP-B-4565036. Hereinafter, referred to as “Patent Literature 2”).

Each of the above-mentioned conventional techniques makes use of a highvoltage charged across a smoothing capacitor originally included in theinverter as a power source for measurement. Therefore, it is notnecessary to separately provide a dedicated power source formeasurement, and therefore, the configuration is simple and eachtechnique is an excellent method in that measurement results with a highaccuracy are obtained because a high measurement voltage is obtained.

In order to measure a high insulation resistance value with a highaccuracy, it is advantageous to increase a measurement current value byincreasing a voltage that is applied. This is obvious from the fact thatmany measuring instruments for measuring an insulation resistance, whichis called an insulation resistance meter or a megohm tester, set highmeasurement voltages, such as 250 [V], 500 [V], and 1,000 [V].

FIG. 1 illustrates an example of a configuration of a motor drive devicethat uses the conventional technique disclosed in Patent Literature 2.

The measurement procedure of the insulation resistance of a motor thatmakes use of a conventional motor drive device 1000 is as follows.First, an alternating-current power source 1002 is disconnected from arectification circuit 1003 by turning off a first switch 1001 in a statewhere all semiconductor switching elements 1051 to 1056 of an inverter1005 including the semiconductor switching elements 1051 to 1056 anddiodes 1051 d to 1056 d connected in inversely parallel thereto areturned off. Next, a second switch 1009 and a third switch 1010 areturned on and a plus side terminal 1042 of a smoothing capacitor 1041 ofa DC link unit 1004 is connected to the ground. As a result of that, acharged voltage of the capacitor 1041 of the DC link unit 1004 isapplied between coils 1061 to 1063 of a motor 1006 and the ground. Atthis time, a current flowing through a closed circuit indicated by adotted line (see FIG. 1) formed by the capacitor 1041, the motor coil(e.g., 1062), and the ground is measured by a current measurementcircuit 1007 provided between the motor coil 1062 and a minus sideterminal 1043 of the capacitor 1041 of the DC link unit 1004. At thesame time as this, a voltage between terminals of the capacitor 1041 ofthe DC link unit 1004 at this time is also measured by a voltagemeasurement circuit 1008 connected in parallel to the DC link unit 1004by using a detection resistor 1081 and a voltage division resistor 1082.Then, the insulation resistance value between the motor 1006 and theground is found from the voltage value and the current value obtained bythe above measurement.

FIG. 2 illustrates an equivalent circuit representing the relationshipof connection between the closed circuit and the semiconductor switchingelement at the time of measurement of the insulation resistance in theconfiguration in FIG. 1 in relation to the conventional motor drivedevice. At the time of measurement, the first switch 1001 is in the OFFstate, and therefore, the alternating-current power source 1002 isdisconnected. Further, the second switch 1009 and the third switch 1010are in the ON state, and therefore, the plus side terminal 1042 of theDC link unit 1004 is connected to the ground and the current measurementcircuit 1007 is connected to the minus side terminal 1043 of the DC linkunit 1004. “RU_(−IGBT)” represents an equivalent insulation resistancevalue when the semiconductor switching elements 1051, 1053, and 1055 ofan upper arm of the inverter are OFF, “RD_(−IGBT)” represents anequivalent insulation resistance value when the semiconductor switchingelements 1052, 1054, and 1056 of a lower arm of the inverter are OFF,“Rm” represents an insulation resistance value between the coil of themotor to be measured and the ground, and “RC” represents a resistancevalue when the serial connection of a voltage division resistor 1072 anda current detection resistor 1071 of the current measurement circuit1007 is represented by one resistor, respectively.

In the conventional technique, due to high voltage charged across thesmoothing capacitor 1041 a leakage current that flows through thesemiconductor switching elements 1051 to 1056 in the OFF state of theinverter 1005 occurs and these currents overlap the measurement current,and therefore, there has been a problem that the measurement accuracy isreduced at high temperatures particularly when the leakage currentflowing through the semiconductor switching elements increases.

In the above description, the “leakage current flowing through thesemiconductor switching elements in the OFF state” refers to, in theexample of an IGBT, a leakage current that flows from the collector toemitter in the state where the IGBT is OFF.

The leakage current in the OFF state is specified as the electricalcharacteristics represented by a symbol I_(CES) in the IGBT and iscalled a “collector-emitter leakage current”. The collector-emitterleakage current (I_(CES)) is specified as a leakage current that flowsfrom the collector to emitter when a specified voltage (usually, amaximum rated voltage) is applied between the collector and emitter inthe state where the gate and emitter are short-circuited, i.e., in thestate where the IGBT is perfectly turned off.

The collector-emitter leakage current (I_(CES)) of the IGBT has strongtemperature dependence and the leakage current I_(CES) has thecharacteristics that the leakage current I_(CES) increases exponentiallywhen temperature rises.

Further, it is known that such characteristics that the leakage currentin the OFF state increases as temperature rises are observed not only inthe IGBT but also in other semiconductor switching elements, such asMOS-FET. For example, in the case of the MOS-FET, the characteristicsare specified as the electrical characteristics represented by a symbolI_(DSS) as the drain-source leakage current in the OFF state.

In general, an increase in the leakage current I_(CES) at hightemperatures in the IGBT for the use as an inverter for driving a motoris regarded as a problem mainly from a viewpoint of an increase in loss.However, even if the leakage current I_(CES) is on the order of severalten [μA], which does not result in any problem from the viewpoint of aloss in a motor drive device, the leakage current I_(CES) will cause areduction in the measurement accuracy in the insulation resistancemeasurement of a motor in the conventional technique.

Specifically, as is obvious from FIG. 2, the problematic point of theconventional technique is that part of the leakage current flowingthrough the semiconductor switching elements 1051 to 1056 in the OFFstate overlaps the current (see a current I indicated by a dotted linearrow in FIG. 2), which is the original target of measurement, flowingthrough the insulation resistor Rm between the motor 1006 and the ground(see FIG. 1), and flows directly into the current measurement circuit1007 (see I_(LEAK) indicated by an alternate long and short dash line inFIG. 2), and therefore, the leakage current flowing through thesemiconductor switching elements 1051 to 1056 directly causes ameasurement error.

In the conventional technique also, if the current flowing through thesemiconductor switching elements 1051 to 1056 is so sufficiently smallthat it can be ignored compared to the measurement current, it isunlikely that the measurement accuracy of the insulation resistancemeasurement of the motor 1006 reduces to bring about a practicalproblem.

A criterion to determine whether or not the equivalent insulationresistance value of the semiconductor switching elements 1051 to 1056 inthe OFF state affects the measurement accuracy of the insulationresistance measurement of the motor can be considered as follows. If theequivalent insulation resistance value of the semiconductor switchingelements 1051 to 1056 in the OFF state is sufficiently large compared tothe insulation resistance value of the motor 1006 to be measured, it canbe considered that no problematic influence will occur. However, in thecase where the equivalent insulation resistance value of thesemiconductor switching elements 1051 to 1056 is equal to or less thanthe insulation resistance value of the motor 1006 to be measured, itwill be difficult to perform insulation resistance measurement withsubstantially high accuracy. This is also obvious from the equivalentcircuit in FIG. 2.

FIG. 3 is a graph indicating the relationship (temperature dependence)between the collector-emitter leakage current I_(CES) [μA], which is theleakage current when the IGBT having a typical withstand voltage of1,200 [V], used in an industrial inverter, and a junction temperatureT_(j) [° C.].

FIG. 3 is a graph obtained by measuring the leakage current in aparallel connection in which the three collectors and the three emittersof the upper arm of the IGBT are connected by supposing a case where theIGBT is used in a three-phase inverter. The graph obtained bymeasurement by similarly parallelly connecting the three collectors andthe three emitters of the lower arm of the IGBT perfectly agree with thegraph of the upper arm, and therefore, in FIG. 3, only one graph isillustrated.

A value obtained by dividing a voltage of 1,200 [V] applied between thecollector and emitter at the time of measurement by the leakage currentI_(CES) [μA] that flows from the collector to emitter, which is readfrom the graph in FIG. 3, is the equivalent insulation resistance valuebetween the collector and emitter at each temperature of the IGBT. Basedon the graph in FIG. 3, to which extent the leakage current of the IGBTaffects the insulation resistance measurement of the conventionaltechnique at each temperature is explained below.

At room temperature (25 [° C.], the leakage current when the IGBT is OFFis as small as about 0.3 [μA] and this corresponds to about 4 [GΩ] interms of the equivalent insulation resistance value of the IGBT. Thisvalue is a sufficiently large value compared to the insulationresistance value (100 [MΩ] to 1 [MΩ]) of the motor to be measured, andtherefore, it can be considered that the leakage current of the IGBTdoes not considerably affect the measurement accuracy of the insulationresistance of the motor at room temperature.

However, as the temperature of the IGBT rises, the leakage current ofthe IGBT increases exponentially. In the case where the conjunctiontemperature T is 80 [° C.], the leakage current of the IGBT is about 40[μA] and this means that in terms of the equivalent insulationresistance value of the IGBT, the value reduces to about 30 [MΩ]. Inthis case, it can be concluded that the equivalent insulation resistancereduces to a level that affects the measurement accuracy due to theleakage current of the IGBT when measuring the insulation resistancevalue of the motor by the conventional technique.

Further, when the conjunction temperature T_(j) rises up to 100 [° C.],the leakage current when the IGBT is OFF increases to about 200 [μA] andin terms of the equivalent insulation resistance value of the IGBT, thevalue is about 6 [MΩ]. In this case, the equivalent insulationresistance value reduces to a resistance value equal to or less than theinsulation resistance value of the motor to be measured, and therefore,it will become difficult to perform insulation resistance measurementwith substantially a high accuracy.

As explained above, in the case where the IGBT having thecharacteristics as illustrated in FIG. 3 is used, the temperature rangein which it is possible to detect insulation deterioration of a motorwith a high accuracy by the conventional technique is limited the rangeof temperatures in the vicinity of room temperature or lower than roomtemperature and it is known that in the state where the temperature ishigh (e.g., immediately after running a motor by an inverter etc.), sucha problem will occur that the accuracy in the insulation resistancemeasurement of a motor and in the detection of insulation deteriorationis degraded considerably because of the influence of the leakage currentof the semiconductor switching elements.

As explained above, the leakage current flowing through thesemiconductor switching elements of the inverter connected to both themotor winding (coil) and the DC link unit overlaps the measurementcurrent, and therefore, particularly at the time of high temperatureswhen the leakage current of the semiconductor switching elementsincreases, there has been such a problem that the accuracy in theinsulation resistance measurement of a motor reduces because of theinfluence of the leakage current of the semiconductor switchingelements.

The present invention has been made in view of these problems and anobject of the present invention is to provide a motor drive device andan insulation resistance detection method of a motor that implementexact measurement of the insulation resistance value and the insulationdeterioration detection of a motor with a simple configuration bysecurely eliminating the influence of the leakage current flowingthrough semiconductor switching elements included in an inverter alsowhen the temperature is high while using a high voltage charged across asmoothing capacitor of a DC link unit originally included in theinverter as a power source.

SUMMARY OF THE INVENTION

A motor drive device according to an embodiment of the present inventionincludes: a rectification circuit configured to rectify analternating-current voltage supplied from an alternating-current powersource via a first switch into a direct-current voltage; a power sourceunit configured to smooth a direct-current voltage rectified by therectification circuit by a capacitor; an inverter unit configured todrive a motor by converting a direct-current voltage smoothed by thepower source unit into an alternating-current voltage by a switchingoperation of a semiconductor switching element; a current detection unitconfigured to measure a value of a current flowing through a resistorone end of which is connected to a coil of the motor and the other endof which is connected to one terminal of the capacitor; a second switchthat grounds the other terminal of the capacitor; and an insulationresistance detection unit configured to stop the operation of the motor,to turn off the first switch, and to detect a value of an insulationresistance of the motor, which is a resistance between the coil of themotor and the ground, by using two sets of a current value and a voltagevalue measured in two states, i.e., a state where the second switch isturned off and a state where the second switch is turned on.

An insulation resistance detection method of a motor according to anembodiment of the present invention includes: a step of, by arectification circuit, rectifying an alternating-current voltagesupplied from an alternating-current power source via a first switchinto a direct-current voltage; a step of, by a power source unit,smoothing a direct-current voltage rectified by the rectificationcircuit by a capacitor; a step of, by an inverter unit, driving a motorby converting a direct-current voltage smoothed by the power source unitinto an alternating-current voltage by a switching operation of asemiconductor switching element; a step of, by a current detection unit,measuring a value of a current flowing through a resistor one end ofwhich is connected to a coil of the motor and the other end of which isconnected to one terminal of the capacitor; a step of, by a voltagedetection unit, measuring a voltage across both ends of the capacitor; astep of providing a second switch that grounds the other terminal of thecapacitor; a step of stopping the operation of the motor and turning offthe first switch; a step of turning off the second switch and measuringa current value and a voltage value; a step of turning on the secondswitch and measuring a current value and a voltage value; and a step ofdetecting a value of an insulation resistance, which is a resistancebetween the coil of the motor and the ground, by using two sets of acurrent value and a voltage value measured in two states, i.e., a statewhere the second switch is turned off and a state where the secondswitch is turned on.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reading the following detailed description, takentogether with the drawings wherein:

FIG. 1 is a configuration diagram of a conventional motor drive device;

FIG. 2 is an equivalent circuit of a closed circuit and a semiconductorswitching element at the time of the insulation resistance measurementof the conventional motor drive device;

FIG. 3 is a graph indicating a temperature dependence of acollector-emitter leakage current when an IGBT is OFF;

FIG. 4 is a configuration diagram of a motor drive device according to afirst embodiment of the present invention;

FIG. 5 is a flowchart illustrating a processing procedure of aninsulation deterioration detection method using the motor drive deviceaccording to the first embodiment of the present invention;

FIG. 6 is an equivalent circuit diagram at the time of (first-time)measurement in the case where a second switch is turned off using themotor drive device according to the first embodiment of the presentinvention;

FIG. 7 is an equivalent circuit diagram at the time of (second-time)measurement in the case where the second switch is turned off using themotor drive device according to the first embodiment of the presentinvention;

FIG. 8 is a configuration diagram of a motor drive device according to asecond embodiment of the present invention;

FIG. 9 is a specific configuration diagram of a circuit of a voltagedetection unit of a converter unit and a circuit of a current detectionunit of an inverter unit in FIG. 8; and

FIG. 10 is a configuration diagram of a higher-level controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, with reference to the drawings, a motor drive device and aninsulation resistance detection method of a motor according to thepresent invention are explained. However, it should be noted that thetechnical scope of the present invention is not limited to embodimentsbelow but encompasses the inventions and equivalents thereof describedin the claims.

First Embodiment

FIG. 4 illustrates a configuration of a motor drive device according toa first embodiment of the present invention.

A motor drive device 101 according to the first embodiment of thepresent invention includes: a rectification circuit 3 configured torectify an alternating-current voltage supplied from analternating-current power source 2 via a first switch 1 into adirect-current voltage; a power source unit 4 configured to smooth adirect-current voltage rectified by the rectification circuit 3 by acapacitor 41; an inverter unit 5 including semiconductor switchingelements 51 to 56 and diodes 51 d to 56 d connected in inverselyparallel thereto and configured to drive a motor 6 by converting adirect-current voltage smoothed by the power source unit 4 into analternating-current voltage by a switching operation of thesemiconductor switching elements 51 to 56; a current detection unit 7configured to measure a value of a current flowing through resistors 71and 72 one end of which is connected to coils 61 to 63 of the motor 6and the other end of which is connected to one terminal 43 of thecapacitor 41; a voltage detection unit 8 configured to measure a valueof a voltage across both ends of the capacitor 41; a second switch 9that grounds the other terminal 42 of the capacitor 41; and aninsulation resistance detection unit 10 configured to stop the operationof the motor 6, to turn off the first switch 1, and to detect a value ofan insulation resistance, which is a resistance between the coils 61 to63 of the motor and the ground, by using two sets of a current value anda voltage value measured in two states, i.e., a state where the secondswitch 9 is turned off and a state where the second switch 9 is turnedon.

The insulation resistance measurement of a motor is performed asfollows. FIG. 5 illustrates a flowchart for explaining the processingprocedure of the insulation deterioration detection method using themotor drive device according to the first embodiment of the presentinvention. First, at step S101, the operation of the motor 6 is stoppedand all the semiconductor switching elements 51 to 56 of the inverter 5are brought into the OFF state in order to measure the insulationresistance of the motor.

Next, at step S102, the alternating-current power source 2 is shut offby turning off the first switch 1. Next, at step S103, in the statewhere the second switch 9 that connects the plus side terminal 42, whichis one end of the capacitor 41, to the ground is turned off, the voltagedetection unit 8 measures the voltage across both ends of the capacitor41 and the current detection unit 7 measures the current flowing throughthe detection resistor 71 that connects one end of the motor coil andthe minus side terminal 43, which is the other end of the capacitor 41.At this time, the first-time measurement is performed in which thevoltage and the current are measured at the same timing by causing thevoltage detection unit 8 and the current detection unit 7 to operate atthe same timing. By measuring the value of the voltage across both endsof the capacitor 41 and the current value of the current detection unit7 at the same timing as described above, the first-time measurement forfinding the leakage current flowing through the semiconductor switchingelements and the equivalent insulation resistance value of thesemiconductor switching elements is performed. Next, at step S104,processing to store the results of the first-time measurement in amemory 11 is performed.

In the conventional technique, a switch, like the third switch 1010 inFIG. 1, for connecting and disconnecting the current measurement circuit1007 is provided, but in the motor drive device 101 according to thefirst embodiment of the present invention, by increasing the resistancevalue of the voltage division resistor 72 of the current detection unit7 and by reducing the current flowing through the current detection unit7 when the motor 6 is in operation to a level that does not affect theoperation of the motor 6, a switch that disconnects the currentdetection unit 7 is not provided and the current detection unit 7 isconnected at all times.

The first-time measurement is measurement in the state where the secondswitch 9 is turned off, and therefore, the current does not flow to theground but only flows from the plus side terminal 42 to the minus sideterminal 43 of the capacitor through the semiconductor switchingelements 51 to 56 and the resistors 71 and 72 of the current detectionunit 7. Consequently, it is possible to represent an equivalent currentat the time of the first-time measurement as in FIG. 6.

In FIG. 6, two upper and lower resistors R_(−IGBT) connected in seriesrepresent the insulation resistance value when the semiconductorswitching elements 51, 53, and 55 of the upper arm of the inverter 5 areOFF and the insulation resistance value when the semiconductor switchingelements 52, 54, and 56 of the lower arm are OFF, respectively, and RCrepresents the serial connection of the voltage division resistor 72 andthe detection resistor 71 of the current detection unit 7 as oneresistor.

The insulation resistance when the semiconductor switching elements 51to 56 are OFF means, in the case where the IGBT is used as thesemiconductor switching element, the equivalent insulation resistancebetween the collector and emitter of the IGBT in the OFF state, which isobtained by dividing the voltage that is applied between the collectorand emitter of the IGBT in the state where the IGBT is turned off by theleakage current that flows from the collector to emitter in the OFFstate.

In FIG. 4, the example is illustrated in which a three-phase motor isused as the motor 6. In the case of the inverter that drives athree-phase motor, there exist three serial connections of two sets ofsemiconductor switching elements of the upper arm and the lower armconnected to both ends of the capacitor 41 as a result. However, each ofthe connection points of the semiconductor switching elements of theupper arm and the lower arm is connected between terminals of each phaseof the motor and the three terminals of each phase of the motor areconnected to one another through the motor coils 61 to 63. Because ofthis, it can be considered that the upper R_(−IGBT) is the combinedresistance when the three power switching elements of the upper arm areconnected in parallel and the lower R_(−IGBT) is the combined resistancewhen the three power switching elements of the lower arm are connectedin parallel.

Further, as the two sets of semiconductor switching elements connectedin series of the upper arm and the lower arm, those having the sameelectrical characteristics are used in the inverter 5 usually. Becauseof this, as explained by using FIG. 3, it can be considered that theequivalent insulation resistance value of the semiconductor switchingelements of the upper arm is equal to that of the semiconductorswitching elements of the lower arm.

In the equivalent circuit in FIG. 6, if a DC link voltage V_(dc1) acrossboth ends of the capacitor 41 that is measured by the voltage detectionunit 8, a current I₁ flowing through the current detection resistor RCthat is measured by the current detection unit 7, and a voltage V_(in1)across both ends of the RC are known from the results of themeasurement, the resistance value of the RC is already known, andtherefore, by applying Kirchhoff's law while paying attention on thecurrent that flows in and out at a P point, it is possible to find theequivalent insulation resistance value R_(−IGBT) of the semiconductorswitching element by the calculation of an arithmetic operation unit 12.

Specifically, as illustrated in FIG. 6, by applying Kirchhoff's firstlaw at the node P, it is possible to calculate the equivalent insulationresistance value R_(−IGBT). First, the current flowing through the RC ofthe current measurement circuit is taken to be I₁, the current flowingthrough the IGBT of the upper arm is taken to be I_(1a), and the currentflowing through the IGBT of the lower arm is taken to be I_(1b). Byapplying Kirchhoff's first law at the node P, a relational expressionbelow is obtained because the sum of currents flowing in from all thebranches connected to one node is zero.I _(1a) −I _(1b) −I ₁=0  (1)

If I_(1a), I_(1b), and I₁ in the above-described expression (1) arerepresented by using the measured voltages V_(dc1) and V_(in1) and eachresistance value R_(−IGBT), and the RC, an expression (2) below isobtained.

$\begin{matrix}{{\frac{\left( {V_{{dc}\; 1} - V_{{in}\; 1}} \right)}{R_{- {IGBT}}} - \frac{V_{{in}\; 1}}{R_{- {IGBT}}} - \frac{V_{{in}\; 1}}{RC}} = 0} & (2)\end{matrix}$By simplifying the expression (2), an expression (3) for calculating theR_(−IGBT) is obtained.

$\begin{matrix}{{\therefore R_{- {IGBT}}} = {\frac{\left( {V_{{dc}\; 1} - {2V_{{in}\; 1}}} \right)}{V_{{in}\; 1}}{RC}}} & (3)\end{matrix}$

The voltage V_(dc1) across both ends of the capacitor 41 obtained by thefirst-time measurement, the current I₁ flowing through the currentdetection resistor RC that is measured by the current detection unit 7,the voltage V_(in1) across both ends of the RC, and the insulationresistance value of R_(−IGBT) obtained as the result of calculation bythe arithmetic operation unit 12 are stored in the memory 11 becausethese are used later to find the insulation resistance of the motor.

Next, at step S105, in the state where the second switch 9 that connectsthe plus side terminal 42, which is one end of the capacitor 41, to theground is turned on, the voltage charged across the capacitor 41 isapplied between the coils 61 to 63 of the motor and the ground and thusa current is caused to occur, which flows through a closed circuitformed by the capacitor 41, the coils 61 to 63 of the motor, and theground. In this state, by using the voltage detection unit 8 thatmeasures the voltage across both ends of the capacitor 41 and thecurrent detection unit 7 that measures the current flowing through thedetection resistor 71 that connects one end of the coils 61 to 63 of themotor and the minus side terminal 43, which is the other end of thecapacitor, second-time measurement for finding the insulation resistancevalue between the coils 61 to 63 of the motor and the ground isperformed by measuring the voltage value across both ends of thecapacitor 41 and the current value of the current detection unit 7 atthe same timing. In the second-time measurement, the voltage and thecurrent are measured at the same timing by causing the voltage detectionunit 8 and the current detection unit 7 to operate at the same timing.After the second-time measurement, the second switch 9 is returned tothe OFF state at step S106.

It is possible to represent the equivalent circuit at the time of thesecond-time measurement as in FIG. 7. Rm in FIG. 7 represents theinsulation resistance, which is to be found, between the coils 61 to 63of the motor and the ground. As in FIG. 6, “R_(−IGB T)” represents theequivalent insulation resistance value when the semiconductor switchingelements 51 to 56 are OFF, and “RC” represents the serial connection ofthe detection resistor 71 and the voltage division resistor 72 of thecurrent detection unit 7 by one resistor. The circuit in FIG. 7 is theequivalent circuit in FIG. 6 to which the connection of the insulationresistor Rm between the ground and the motor coil is added. It is knownthat at the time of the second-time measurement, in the currentdetection unit 7, the current value that combines the value of thecurrent flowing through the closed circuit formed by the capacitor 41,the coils 61 to 63 of the motor, and the ground and the value of part ofthe leakage current flowing through the semiconductor switching elements51 to 56 in the OFF state is measured.

Next, at step S107, the results of the second-time measurement arestored in the memory and the processing to perform an arithmeticoperation to find the insulation resistance value from the results ofthe first-time measurement and the second-time measurement,respectively, is performed. In the equivalent circuit in FIG. 7, if a DClink voltage V_(dc2) across both ends of the capacitor, which ismeasured by the voltage detection unit 8, a current I₂ flowing throughthe current detection resistor RC, which is measured by the currentdetection unit 7, and a voltage V_(in2) across both ends of the RC areknown from the results of the measurement, the resistance value of theRC is already known and the equivalent insulation resistance valueR_(−IGBT) of the semiconductor switching elements 51 to 56 that has beenfound by the arithmetic operation from the results of the previousmeasurement is also known, and therefore, it is possible to find theinsulation resistance Rm between the motor and the ground bycalculation, which is the only one unknown value in the equivalentcircuit in FIG. 7 in which four resistors are connected in the form ofan H bridge.

It is possible to calculate the insulation resistance Rm as follows byapplying Kirchhoff's first law at the node P and substituting acalculation expression that has been found from the results of thefirst-time measurement in the value of R_(−IGBT). First, as illustratedin FIG. 7, the current flowing through the RC of the current measurementcircuit is taken to be I₂, a current flowing through the IGBT of theupper arm to be I_(2a), a current flowing through the IGBT of the lowerarm to be I_(2b), and a current flowing through the insulation resistorRm of the motor to be I_(2c). If Kirchhoff's first law is applied at thenode P, the sum of the currents that flow in from all the branchesconnected to one node is zero, and therefore, a relational expressionbelow is obtained.I _(2a) +I _(2c) −I _(2b) −I ₂=0  (4)

If I_(2a), I_(2b), I_(2c), and I₂ in the above-described expression (4)are represented by using the measured voltages V_(dc2) and V_(in2) andeach resistance value R_(−IGBT), the Rm, and the RC, an expression (5)below is obtained.

$\begin{matrix}{{\frac{\left( {V_{{dc}\; 2} - V_{{in}\; 2}} \right)}{R_{- {IGBT}}} + \frac{\left( {V_{{dc}\; 2} - V_{{in}\; 2}} \right)}{Rm} - \frac{V_{{in}\; 2}}{R_{- {IGBT}}} - \frac{V_{{in}\; 2}}{R\; C}} = 0} & (5)\end{matrix}$

By simplifying the expression (5), an expression (6) below is obtained.

$\begin{matrix}{{\frac{\left( {V_{{dc}\; 2} - {2V_{{in}\; 2}}} \right)}{R_{- {IGBT}}} + \frac{\left( {V_{{dc}\; 2} - V_{{in}\; 2}} \right)}{Rm} - \frac{V_{{in}\; 2}}{RC}} = 0} & (6)\end{matrix}$

By substituting the expression (3) for finding R_(−IGBT) that has beenfound by the first-time measurement in the above-described expression(6) and by simplifying the expression (6), an expression (7) below isobtained.

$\begin{matrix}{{\frac{\left( {V_{{dc}\; 2} - {2V_{{in}\; 2}}} \right)V_{{in}\; 1}}{\left( {V_{{dc}\; 1} - {2V_{{in}\; 1}}} \right){RC}} + \frac{\left( {V_{{dc}\; 2} - V_{{in}\; 2}} \right)}{Rm} - \frac{V_{{in}\; 2}}{RC}} = 0} & (7)\end{matrix}$

By simplifying the expression (7), an expression (8) below is obtained.

$\begin{matrix}{{\frac{\left( {{V_{{dc}\; 2}V_{{in}\; 1}} - {V_{{dc}\; 1}V_{{in}\; 2}}} \right)}{\left( {V_{{dc}\; 1} - {2V_{{in}\; 1}}} \right){RC}} + \frac{\left( {V_{{dc}\; 2} - V_{{in}\; 2}} \right)}{Rm}} = 0} & (8)\end{matrix}$

By using the equation (8), an equation (9) for finding the insulationresistance Rm of the motor is obtained from two sets of the currentvalue and the voltage value including the results V_(dc1) and V_(in1) ofthe first-time measurement and the results V_(dc2) and V_(in2) of thesecond-time measurement, and the already-known resistance value RC ofthe current detection resistor.

$\begin{matrix}{{Rm} = {\frac{\left( {V_{{dc}\; 1} - {2V_{{in}\; 2}}} \right)\left( {V_{{dc}\; 2} - V_{{in}\; 2}} \right)}{\left( {{V_{{dc}\; 1}V_{{in}\; 2}} - {V_{{dc}\; 2}V_{{in}\; 1}}} \right)}{RC}}} & (9)\end{matrix}$

By sequentially performing the above series of processing, it ispossible to detect the insulation resistance value Rm of the motor.Further, at step S108, it is also possible to add a step, which isperformed by an insulation deterioration determination unit 14, ofperforming processing to give a warning or an alarm to report the degreeof insulation deterioration obtained by comparing the obtainedinsulation resistance value of the motor with a reference value.Furthermore, at step S109, it is also possible to add processing torecord and display the insulation resistance value of the motor obtainedeach time the measurement is completed together with information on thedate and time when the measurement is completed for each motor.

In the flowchart in FIG. 5, it may also be possible to change the orderof the first-time measurement (step S103) and the second-timemeasurement (step S105).

In the manner as described above, by using V_(dc2), I₂, and V_(in2)obtained by the second-time measurement, the results V_(dc1), I₁, andV_(in1) of the first-time measurement stored in the memory 11, andR_(−IGBT) that has been found by the arithmetic operation from theresults of the first-time measurement, the exact insulation resistancevalue Rm of the motor is found by calculation by the arithmeticoperation unit 12, and the insulation resistance value Rm of the motorthat has been found is compared with the reference value in theinsulation deterioration determination unit 14, and then a notificationor a display, such as a warning and an alarm, is given in accordancewith the degree of the reduction in the insulation resistance value ofthe motor based on the results of the comparison.

With regard to the calculation for finding the insulation resistancevalue Rm of the motor in the arithmetic operation unit 12, in theabove-described explanation, the method for calculating the insulationresistance value Rm of the motor from the results of the second-timemeasurement by using the value of R_(−IGBT) after temporarily findingthe equivalent insulation resistance value R_(−IGBT) of thesemiconductor switching elements from the results of the first-timemeasurement is explained. However, it may also be possible to directlyfind the insulation resistance Rm between the motor and the ground bycalculation from the values V_(dc1), I₁, and V_(in1) of the first-timemeasurement and the values V_(dc2), I₂, and V_(in2) of the second-timemeasurement by regarding the value of R_(−IGBT) as variables of thevalues V_(dc1), I₁, and V_(in1) of the first-time measurement withoutperforming calculation for finding the value of the R_(−IGBT).

Further, with regard to the order of measurement, in the aboveexplanation, the example is described in which the first-timemeasurement is performed in the state where the second switch 9 thatconnects the plus side terminal 42, which is one end of the capacitor41, to the ground is turned off and next, the second-time measurement isperformed in the state where the second switch 9 is turned on. However,it may also be possible to perform measurement in the order opposite tothat in the above-described explanation. In other words, it is possibleto finally perform an arithmetic operation by using measured values ofthe measurement performed twice after storing the measured resultsobtained by the measurement performed twice, and therefore, the order ofmeasurement is not restricted.

Further, the insulation resistance value of the motor that has beenfound by the calculation in the arithmetic operation unit 12 isdelivered to a higher-level controller 15 via the insulation resistancedetection unit 10 as the results of the measurement of the insulationresistance of the motor as well as being used for determination of theinsulation deterioration detection of the insulation deteriorationdetermination unit 14.

In either case, the present invention is characterized in that byperforming measurement twice in the state where the second switch 9 thatconnects the plus side terminal 42, which is one end of the capacitor41, to the ground is turned off and in the state where the second switch9 is turned on, the equivalent insulation resistance of thesemiconductor switching elements in the OFF state, in other words, theinfluence of the leakage current of the semiconductor switching elementsis found exactly by calculation of the results of the measurementperformed twice, and thereby, the exact insulation resistance value ofthe motor after the influence of the leakage current of thesemiconductor switching elements is perfectly eliminated is found.

Further, it may also be possible to further include a display unit 13configured to display the results of the insulation deteriorationdetermination by comparing the insulation resistance value that has beenfound with a reference value set in advance. Furthermore, it may also bepossible to record the results of the measurement of the insulationresistance value of each motor together with information on date andtime of the measurement each time the measurement of the insulationresistance value of the motor is completed, and to display the historyof the recorded date and time and the insulation resistance value of themotor on the display unit 13.

Still furthermore, as illustrated in FIG. 4, it may also be possible tocause the insulation resistance detection unit 10 to transmit theresults of the measurement of the insulation resistance value of themotor 6, a measurement completion signal indicating the timing at whichthe measurement is completed, and the results of the insulationdeterioration determination to the higher-level controller 15.

A configuration example of the higher-level controller 15 is illustratedin FIG. 10. The higher-level controller 15 is numerically controlledequipment that provides instructions to drive the motor 6 to eachinverter 5. The higher-level controller 15 includes a serialcommunication circuit 246 for communicating with each inverter 5, amicrocomputer 17 having a built-in function to check information on dateand time, a nonvolatile ROM 16, and the display unit 13. Further, thehigher-level controller 15 displays the history of the results of themeasurement of the insulation resistance of each motor and theinformation on data and time in the past recorded in the ROM 16 on thedisplay unit 13 as well as recording information in which information ondate and time when the measurement completion signal is received fromthe insulation resistance detection unit 10 and the measured insulationresistance value are caused to correspond to each other in a one-to-onemanner for each motor in the ROM 16.

It is possible for a user to display the transition of the insulationresistance value of each motor 6 at each date and time of themeasurement from the past to the present time on the display unit 13 ofthe higher-level controller 15 by operating the higher-level controller15. Alternatively, it is possible to make use of the data to protect andmaintain a machine that uses the motor drive device 101 because the datacan be read and checked from the ROM 16 within the higher-levelcontroller 15.

Second Embodiment

Next, a motor drive device according to a second embodiment isexplained. FIG. 8 illustrates a configuration of a motor drive device102 according to the second embodiment. In the second embodiment, themotor drive device 102 drives a plurality of motors 6 a and 6 b and itis made possible to perform exact insulation resistance measurement of amotor after eliminating the influence of the leakage currents flowingthrough semiconductor switching elements 51 a to 56 a and 51 b to 56 band to perform insulation deterioration detection.

To the first motor 6 a, a first inverter 5 a including the semiconductorswitching elements 51 a to 56 a and diodes 51 da to 56 da connected ininversely parallel thereto is connected. To the second motor 6 b, asecond inverter 5 b including the semiconductor switching elements 51 bto 56 b and diodes 51 db to 56 db connected in inversely parallelthereto is connected. By performing first-time measurement for findingthe influence of the leakage currents flowing through the semiconductorswitching elements 51 a to 56 a and 51 b to 56 b, respectively, of eachof the first and second inverters 5 a and 5 b, and for finding anequivalent insulation resistance value of the semiconductor switchingelements for each motor, and second-time measurement for finding aninsulation resistance value between coils 61 a to 63 a of the firstmotor and the ground and an insulation resistance value between coils 61b to 63 b of the second motor and the ground, it is possible to find anexact insulation resistance value of the motor after eliminating theinfluence of the leakage currents flowing through the semiconductorswitching elements 51 a to 56 a and 51 b to 56 b for each of the firstand second motors 6 a and 6 b from the results of the measurementperformed twice.

In the second embodiment, the first-time measurement and the second-timemeasurement are performed at a time at the same timing in the commonvoltage detection unit 8 and in first and second current detection units7 a and 7 b of the first and second inverter units 5 a and 5 b,respectively.

The number of times of measurement remains the same even in the casewhere there is a plurality of motors and only by performing measurementtwice in the state where the second switch 9 is turned off and in thestate where the second switch 9 is turned on, it is possible to obtainthe insulation resistance values of all the motors and to performinsulation deterioration detection for each motor.

In the motor drive device 102 according to the second embodimentillustrated in FIG. 8, to one power source unit (converter unit) 20, thefirst inverter unit 5 a configured to drive the first motor 6 a and thesecond inverter unit 5 b configured to drive the second motor 6 b areconnected. Rm1 and Rm2 represent the insulation resistance value betweenthe coils 61 a to 63 a of the first motor 6 a and the ground and theinsulation resistance value between the coils 61 b to 63 b of the secondmotor 6 b and the ground, respectively. In the example in FIG. 8, thefirst inverter unit 5 a and the second inverter unit 5 b are providedwith a smoothing capacitor 41 a and a smoothing capacitor 41 b,respectively, but these two capacitors 41 a and 41 b are connected inparallel, and therefore, they function as one capacitor having anelectrostatic capacitance, which is the total capacitance of the twocapacitors.

FIG. 8 illustrates an example of the motor drive device that drives thetwo motors 6 a and 6 b, but it is needless to say that the number ofmotors is not limited to that in this example in the second embodiment.

In the second embodiment, as illustrated in FIG. 8, the rectificationcircuit 3 configured to rectify an alternating-current voltage suppliedfrom the alternating-current power source 2 via the first switch 1 intoa direct-current voltage, the second switch 9 that connects a plus sideterminal 42 a and a plus side terminal 42 b, which are each one end ofthe capacitor 41 a and the capacitor 41 b, respectively, which smooththe output of the rectification circuit 3, to the ground, and thevoltage detection unit 8 (and an AD converter 21 that converts theoutput thereof into a digital value) configured to measure the voltageacross both ends (42 a and 43 a) of the capacitor 41 a and the voltageacross both ends (42 b and 43 b) of the capacitor 41 b can be sharedbetween the first and second inverters 5 a and 5 b, and therefore, it isnot necessary to have each of those units in plurality for each motorbut only one is sufficient.

In contrast to this, the first and second current detection units 7 aand 7 b (and AD converters 73 a and 73 b that convert the outputsthereof into digital values) exist in plurality for the pair of thefirst motor 6 a and the first inverter 5 a that drives the motor 6 a andfor the pair of the second motor 6 b and the second inverter 5 b thatdrives the motor 6 b, respectively, and as the current values, thevalues measured by the first and second current detection units 7 a and7 b corresponding to each motor are used and as the voltage value, thevalue measured by the single voltage detection unit 8 in common to allthe motors is used. By this configuration, it is possible to exactlyfind the influence of the leakage current flowing through thesemiconductor switching elements 51 a to 56 a in the OFF state and theinfluence of the leakage current flowing through the semiconductorswitching elements 51 b to 56 b for each of the pair of the motor andthe first inverter 5 a that drives the motor and the pair of the motorand the second inverter 5 b that drives the motor, and the equivalentinsulation resistance value of the semiconductor switching elements foreach motor.

As described above, in the conventional technique, a switch, like thethird switch 1010 in FIG. 1, which connects and disconnects the currentmeasurement circuit of each inverter 1005 was provided. In contrast tothis, in the present invention, as illustrated in FIG. 8, by increasingthe resistance values of voltage division resistors 72 a and 72 b of thefirst and second current detection units 7 a and 7 b to reduce thecurrents flowing through the first and second current detection units 7a and 7 b when the motors are in operation to values that will notaffect the operation of the motors, switches to disconnect the first andsecond current detection units 7 a and 7 b are not provided and thefirst and second current detection units 7 a and 7 b are left connectedat all times. Due to this, the switches of the current detection units,which were necessary in the number corresponding to that of theinverters in the conventional technique, are removed and a simple andlow-cost insulation resistance detection unit is implemented.

FIG. 9 illustrates a specific configuration example of the circuit ofthe voltage detection unit 8 of the converter unit 20 and the circuit ofthe first and second current detection units 7 a and 7 b of the firstand second inverter units 5 a and 5 b in FIG. 8. Each of the circuits isa circuit configured to measure a voltage Vs that appears across bothterminals of the detection resistor and the resistance values of adetection resistor 81 (or 71 a, 71 b) and a voltage division resistor 82(72 a, 72 b) are already known, and therefore, in the first and secondinverter units 5 a and 5 b, the circuit is used as a current measurementcircuit for finding a value of a current Id flowing through thedetection resistor 81 (or 71 a, 71 b) from the measurement results, andin the converter unit, the circuit is used as a voltage measurementcircuit for finding a voltage across both ends of the serial connectionof the voltage division resistor 82 and the detection resistor 81 fromthe voltage division ratio of the resistor.

Both the detection resistor of the circuit of the voltage detection unit8 of the converter unit 20 and the detection resistors of the circuitsof the first and second current detection units 7 a and 7 b of the firstand second inverter units 5 a and 5 b are connected to the circuits onthe primary side. Because of this, the detected voltage converted into asecondary potential by using an insulation amplifier 22 is input to theAD converter 21 (or 73 a, 73 b) and thus is converted into a digitalvalue.

“A/D” in FIG. 8 indicates an “A/D converter” and is the same as the “A/Dconverter” in FIG. 9.

In FIG. 8, the “insulation resistance detection unit” in FIG. 4 isimplemented by microcomputers A to C (23, 74 a, 74 b). By themicrocomputers A to C (23, 74 a, 74 b) giving instructions atappropriate timing in accordance with the flowchart as illustrated inthe example in FIG. 5, processing necessary for measurement isimplemented, such as the operation to turn off the semiconductorswitching elements 51 a to 56 a and 51 b to 56 b of the first and secondinverter units 5 a, 5 b, the operation to turn ON/OFF the first switch 1and the second switch 9, and the AD conversion operation of the ADconverters 21, 73 a, and 73 b for taking in the measured values of thevoltage detection unit 8 and the first and second current detectionunits 7 a and 7 b.

In FIG. 8, the “memory 11”, the “arithmetic operation unit 12”, and the“insulation deterioration determination unit 14” in FIG. 4 are alsoimplemented by the microcomputers A to C (23, 74 a, 74 b). Themicrocomputers A to C (23, 74 a, 74 b) read the measurement results ofthe voltage detection unit 8 and the first and second current detectionunits 7 a and 7 b as digital values from the AD converters 21, 73 a, and73 b, respectively. In the present invention, the measurement isperformed twice and each measured value is held until the finalarithmetic operation is performed inside the microcomputers A to C (23,74 a, 74 b) and by using the measured values held after the second-timemeasurement, the insulation resistance value of the motor is found bythe arithmetic operation of the microcomputers B and C (74 a, 74 b). Theprocessing to determine insulation deterioration by comparing theinsulation resistance value of the motor that has been found with thereference value and to report the determination results to the outsideis also performed in the microcomputers B and C (74 a, 74 b).

In FIG. 8, one microcomputer A (23) is provided in the common converterunit 20 and is provided in the first inverter unit 5 a and in the secondinverter unit 5 b, respectively, and the respective microcomputers areconnected with one another by serial communication. This configurationis a configuration example suitable to the case where the converterunit, the first inverter unit 5 a, and the second inverter unit 5 b haveindividual separate casings.

In the case where all of the converter units 20 and the first and secondinverter units 5 a and 5 b are included within one and the same casing,it may be possible to omit the serial communication circuit between thecomputers by causing one microcomputer to perform processing. In thecase where only the first and second inverter units 5 a and 5 b areincluded within one and the same casing, it may be possible to cause onemicrocomputer to perform processing of the first and second inverterunits 5 a and 5 b within the same casing.

In the case where a plurality of microcomputers is used for theinsulation resistance measurement of the first and second motors 6 a and6 b as in the embodiment in FIG. 8, the insulation resistancemeasurement can be implemented by causing any one of the microcomputersto function as a master in the operation of the insulation resistancemeasurement of the first and second motors 6 a and 6 b, and the othermicrocomputers to function as slaves, and by causing the mastermicrocomputer to give instructions to perform the operation to the slavemicrocomputers.

In the present embodiment, the case is explained where the microcomputerA (23) of the converter unit 20 is caused to function as a master. Themicrocomputer A (23) of the converter unit 20 gives instructions to turnoff all the IGBTs 51 a to 56 a and 51 b to 56 b of the first and secondinverter units 5 a and 5 b to all the first and second inverter units 5a and 5 b via serial communication units 241, 242, and 244. Themicrocomputer B (74 a) and the microcomputer C (74 b) having receivedthe instructions turn off the IGBTs of the inverters of their own. Next,the microcomputer A (23) turns off the first switch.

Next, the microcomputer A (23) performs the first-time measurement inthe state where the second switch 9 is turned off. Specifically, themicrocomputer A (23) gives instructions and notifies all the first andsecond inverter units 5 a and 5 b of the timing at which the first andsecond current detection units 7 a and 7 b should perform measurementvia the serial communication circuits 241, 242, and 244. Themicrocomputer B (74 a) and the microcomputer C (74 b) of the first andsecond inverter units 5 a and 5 b having received the notificationacquire the measured values of the first current detection unit 7 a andthe second current detection unit 7 b from the AD converters 73 a and 73b, respectively. Further, the microcomputer A (23) itself also acquiresthe measured value of the voltage detection unit 8 of the converter unit20 from the AD converter 21 at the same timing as that at which themicrocomputers B and C (74 a, 74 b) of the first and second inverterunits 5 a and 5 b acquire the measured values from the AD converters 73a and 73 b. Each of the microcomputers A to C (23, 74 a, 74 b) is causedto hold and store the measured value acquired from each of the ADconverters 21, 73 a, and 73 b as the result of the first-timemeasurement.

Next, the microcomputer A (23) performs the second-time measurement inthe state where the second switch 9 is turned on. As in the first-timemeasurement, the microcomputer A (23) gives instructions and notifiesall the first and second inverter units 5 a and 5 b of the timing atwhich the first and second current detection units 7 a and 7 b shouldperform measurement via the serial communication circuits 241, 242, and244. The microcomputer B (74 a) and the microcomputer C (74 b) of thefirst and second inverter units 5 a and 5 b having received thenotification acquire the measured values of the first current detectionunit 7 a and the second current detection unit 7 b from the ADconverters 73 a and 73 b, respectively. Further, the microcomputer A(23) itself also acquires the measured value of the voltage detectionunit 8 of the converter unit 20 from the AD converter 21 at the sametiming as that at which the microcomputers B and C (74 a, 74 b) of thefirst and second inverter units 5 a and 5 b acquire the measured valuesfrom the AD converters 73 a and 73 b. Each of the microcomputers A to C(23, 74 a, 74 b) is caused to hold and store the measured value acquiredfrom each of the AD converters 21, 73 a, and 73 b as the result of thesecond-time measurement. When the second-time measurement is completed,the microcomputer A (23) returns the second switch 9 to the OFF state.

When the second-time measurement is completed, the microcomputer A (23)transmits the measured values of the first-time measurement and thesecond-time measurement which have been measured by the voltagedetection unit 8 of the converter unit 20 to all the first and secondinverters 5 a and 5 b via the serial communication circuits 241, 242,and 244.

Upon receipt of the measured values of the first-time measurement andthe second-time measurement of the voltage detection unit 8 of theconverter unit 20 via the serial communication circuits 241, 242, and244, the microcomputer B (74 a) and the microcomputer C (74 b) of thefirst and second inverter units 5 a and 5 b use the measured values ofthe first-time measurement and the second-time measurement of the firstcurrent detection unit 7 a and the second current detection unit 7 bheld by the microcomputer B (74 a) and the microcomputer C (74 b)themselves and the measured values of the first-time measurement and thesecond-time measurement of the voltage detection unit 8 of the converterunit 20 which have been received via the serial communication circuits241, 242, and 244, and the microcomputer B (74 a) calculates theinsulation resistance value Rm1 of the first motor 6 a by an arithmeticoperation and the microcomputer C (74 b) calculates the insulationresistance value Rm2 of the second motor 6 b by an arithmetic operation.

The microcomputer B (74 a) and the microcomputer C (74 b) notify thehigher-level controller 15 of the insulation resistance values of themotors obtained respectively by the arithmetic operation and ameasurement completion signal indicating the timing at which themeasurement is completed via serial communication circuits 243, 245, and246. Further, the microcomputer B (74 a) and the microcomputer C (74 b)also notify the higher-level controller 15 of the results of theinsulation deterioration determination that has determined the degree ofinsulation deterioration by comparing the obtained insulation resistancevalues of the motors with a reference value set in advance via theserial communication circuits 243, 245, and 246.

A configuration example of the higher-level controller 15 is asexplained in FIG. 10. The higher-level controller 15 records informationin which information on the date and time of the reception of themeasurement completion signal and the measurement result of theinsulation resistance value are made to correspond to each other in aone-to-one manner for each of the first and second motors 6 a and 6 b inthe ROM 16 as well as displaying the insulation deteriorationdetermination results having been received via the serial communicationcircuits 243, 245, and 246 on the display unit 13. Further, thehigher-level controller 15 displays the results of the insulationresistance measurement and information on the date and time in the pastfor each motor recorded in the ROM 16.

With regard to the above-described reference value, it is possible toset any value from outside to the microcomputer B (74 a) and themicrocomputer C (74 b) as a plurality of reference values for eachmotor. For example, if two reference values, one of which is 20 [MΩ] asa warning level and the other of which is 2 [MΩ] as an alarm level, areset in advance from outside in accordance with a machine, a device, orthe like, that is used and the motor drive device is designed so that awarning is given as a notification when the insulation resistance valueof the motor which has been obtained as the result of the measurement isequal to or less than 20 [MΩ], and an alarm is given as a notificationwhen it is equal to or less than 2 [MΩ], it is possible for an operatorof the machine etc. to determine the degree of insulation deteriorationof the motor from the contents of the notification even if he/she has noexpertise to determine the degree of insulation deterioration.

Further, in the case where it is desired to set different warning levelsand different alarm levels for different motors, reference values may beset for each motor.

The arithmetic operation unit and the determination unit may be providedin each of the microcomputers B and C (74 a, 74 b) of the first andsecond inverters units 5 a and 5 b for each motor as in FIG. 8 orarithmetic operations and determinations of a plurality of motors may beperformed by one arithmetic unit and one determination unit.

The motor drive device is designed so that the instructions to turn offall the IGBTs 51 a to 56 a and 51 b to 56 b of the first and secondinverter units 5 a and 5 b before measurement, which the microcomputer A(23) of the converter unit 20 transmits to all the first and secondinverter units 5 a and 5 b via the serial communication circuits 241,242, and 244, the instructions to indicate the timing at which the firstand second inverter units 5 a and 5 b are caused to perform the currentmeasurement in the second-time measurement, and the voltage measuredvalues of the converter unit 20 which are sent to the first and secondinverter units 5 a and 5 b after the second-time measurement iscompleted may be received in common by all the first and second inverterunits 5 a and 5 b. Further, the motor drive device is designed so thatupon receipt of instructions or data in common to all the first andsecond inverter units 5 a and 5 b from the converter side 20 via theserial communication circuits 241, 242, and 244, all the first andsecond inverter units 5 a and 5 b perform the determined operationdescribed previously at the determined timing.

In particular, in the case of the motor drive device in which aplurality of inverter units (5 a, 5 b, . . . ) and the converter unit 20are included in separate casings, there can be considered a variety ofcombinations of the number of inverter units (5 a, 5 b, . . . )connected to one converter unit 20 and the number of motors (6 a, 6 b, .. . ) connected via the inverter unit. However, by making use of theabove-described mechanism, it is made possible to measure the insulationresistances of all the motors connected to the same converter unit via aplurality of inverter units and to detect insulation deteriorationwithout the need to perform special settings or to change connectionsfor any combination of the number of inverter units connected to theconverter unit 20 and the number of motors.

According to the present invention, it is possible to detect the stateof deterioration of the insulation resistor of a motor with a highaccuracy compared to the conventional technique.

What is claimed is:
 1. A motor drive device, comprising: a first switch;a rectification circuit configured to rectify an alternating-currentvoltage supplied from an alternating-current power source via the firstswitch into a direct-current voltage; a power source unit comprising acapacitor and configured to smooth the direct-current voltage rectifiedby the rectification circuit by the capacitor; an inverter unitcomprising a semiconductor switching element and configured to drive amotor by converting the direct-current voltage smoothed by the powersource unit into an alternating-current voltage by a switching operationof the semiconductor switching element; a resistor, one end of which isconnected to a coil of the motor and the other end of which is connectedto a first terminal of the capacitor; a current detection unitconfigured to measure a current value of a current flowing through theresistor; a voltage detection unit configured to measure a voltage valueof a voltage across the first terminal and a second terminal of thecapacitor; a second switch that grounds the second terminal of thecapacitor; and an insulation resistance detection unit configured tostop the operation of the motor, turn off the first switch, and detectan insulation resistance value of an insulation resistance of the motor,by using two sets of the current value and the voltage value measured ina state where the second switch is turned off and in a state where thesecond switch is turned on, wherein an influence of a leakage current ofthe semiconductor switching element is eliminated by using the two setsof the current value and the voltage value measured in the state wherethe second switch is turned off and in the state where the second switchis turned on, and wherein the insulation resistance is a resistancebetween the coil of the motor and the ground.
 2. The motor drive deviceaccording to claim 1, further comprising: a memory storing a referencevalue of the insulation resistance of the motor; an arithmetic operationunit configured to compare the insulation resistance value detected bythe insulation resistance detection unit and the reference value set inadvance; and a display unit configured to display a result of thecomparison made by the arithmetic operation unit.
 3. The motor drivedevice according to claim 1, wherein each time the insulation resistanceis measured, the insulation resistance value detected by the insulationresistance detection unit is recorded together with information on thedate and time of the measurement of the insulation resistance.
 4. Aninsulation resistance detection method of a motor, the methodcomprising: rectifying, by a rectification circuit, analternating-current voltage supplied from an alternating-current powersource via a first switch into a direct-current voltage; smoothing, by acapacitor of a power source unit, the direct-current voltage rectifiedby the rectification circuit; driving, by an inverter unit, a motor byconverting the direct-current voltage smoothed by the power source unitinto an alternating-current voltage by a switching operation of asemiconductor switching element of the inverter unit; measuring, by acurrent detection unit, a current value of a current flowing through aresistor one end of which is connected to a coil of the motor and theother end of which is connected to a first terminal of the capacitor;measuring, by a voltage detection unit, a voltage value of a voltageacross the first terminal and a second terminal of the capacitor;stopping the operation of the motor and turning off the first switch;turning off a second switch, which grounds the second terminal of thecapacitor, and measuring the current value and the voltage value toobtain a first set of the current value and the voltage value; turningon the second switch and measuring the current value and the voltagevalue to obtain a second set of the current value and the voltage value;and detecting an insulation resistance value of an insulation resistanceof the motor, by using the first and second sets of the current valueand the voltage value measured when the second switch is turned off andwhen the second switch is turned on, wherein an influence of a leakagecurrent of the semiconductor switching element is eliminated by usingthe two sets of the current value and the voltage value measured in thestate where the second switch is turned off and in the state where thesecond switch is turned on, and wherein the insulation resistance is aresistance between the coil of the motor and the ground.